2.2 Noise outPkTB= (2.5)2.2.3 Available Power from AntennaThe noise from an antenna can be modeled as a resistor . Thus, as in Section 2.2.2, the available power from an antenna is given by: -== ´21available4 10W / HzPkT (2.6)at T = 290K, or in dBm/Hz: --æö´== -ç÷´èø21available1034 1010log174 dBm / Hz1 10P (2.7)Using 290K as the temperature of the resistor modeling the antenna is appropri-ate for cell phone applications where the antenna is pointed at the horizon. How-ever, if the antenna were pointed at the sky, the equivalent noise temperature would be much lower—typically 50K . For any receiver required to receive a given bandwidth of signal, the minimum detectable signal can now be determined. As can be seen from (2.5), the noise floor depends on the bandwidth. For example, with a bandwidth of 200 kHz, the noise floor is: --== ´´= ´2116Noise Floor4 10200,000 8 10WkTB (2.8)More commonly, the noise floor would be expressed in dBm, as in the following for the same example as before: = -+= -10Noise Floor174 dBm / Hz 10log (200,000)121 dBm (2.9)Figure 2.1 Resistor noise model: (a) with a voltage source and (b) with a current source.